A heating device embodying the invention is particularly suitable for indelibly fixing on a sheet of paper, or any any other convenient medium, information transferred either following the development of latent images with a toner or following printing with an ink which has to be dried rapidly. In the case of fixing information developed with a toner, the heating device is applied in the fixing equipment or fuser in a copier or printer of the type comprising a belt having low thermal inertia, generally consisting of an electrical resistance element held by a support disposed perpendicularly with respect to the direction of advance of a copy sheet in the fuser.
European patent application no. 426,072 describes image fixing equipment for a electrical photocopier in which the heating element consists of a principal strip of resistive material whose length is at least equal to the transverse dimension of the largest paper format.
The heating element is heating by applying a suitable direct or alternating voltage to its ends so that the current passing through it generates, by the Joule effect, the amount of heat necessary to fuse and fix the toner on the paper.
When paper formats smaller than the maximum size are used, a portion of the heating element remains uncovered by the paper and is in contact with the fuser advance belt. To avoid losses of electrical energy and possible damage due to the high temperature of the heating element, according to the cited European patent, auxiliary parallel resistive strips are disposed beside the principal strip. These auxiliary strips are connected selectively in parallel with the principal strip at points along its length corresponding to the various paper formats in use. A control circuit is used to connect one or more of the auxiliary strips in parallel with the principal strip, according to,the preselected format.
The heat generated by the principal strip in the part not covered by the paper is thus less than the nominal value required for fixing.
In this arrangement, the number of formats which may be used is limited to 3 or 4, since the width of the support of the auxiliary resistive strips would become excessive. Moreover, the size of each usable format is constant, and it is impossible to use arbitrary paper formats not specified at the design stage.